1. Technical Field
The present invention relates to a valve train for opening and closing engine valves, namely, intake valves or exhaust valves, of an internal combustion engine. More concretely, the present invention relates to a valve train provided with a torque reducing mechanism for suppressing the fluctuation of torque applied to a camshaft provided with valve-operating cams for opening and closing the valves of an engine.
2. Description of the Related Art
A valve train for an internal combustion engine, disclosed in, for example, JP 62-48105 B includes a camshaft provided with cams for opening and closing valves, and a torque reducing mechanism for applying a counter torque for suppressing torque fluctuation resulting from forces applied to the cams by the valves to the camshaft. The torque reducing mechanism includes a rotating member, such as a counteraction cam which rotates together with the camshaft, and a counterforce applying mechanism for applying counterforce for producing the counter torque to the rotating member.
In the valve train including the camshaft rotatably supported on a cylinder head by a plurality of camshaft supports, wherein the torque reducing mechanism is disposed at a position between two adjacent ones of the camshaft supports, the counterforce applying mechanism is disposed at a position where the camshaft support is to be disposed on the cylinder head, and the rotating member, namely, the counteraction cam, is mounted on a part of the camshaft to be supported by the camshaft support, namely, a journal, to suppress increase in the length of the camshaft. With this arrangement, the rigidity of the camshaft needs to be enhanced to prevent augmentation of the bending deformation of the camshaft resulting from the reduction of the number of the camshaft supports, which causes the weight of the camshaft to increase. The counterforce applied by the counterforce applying mechanism to the rotating member tends to augment the bending deformation still further. In such a case the rigidity of the camshaft needs to be enhanced still further, which increases the weight of the camshaft still further.
When the counterforce applying mechanism of the torque reducing mechanism includes a pressing device for producing a pressing force and a counterforce resulting from the pressing force is applied to the rotating member in the same direction as the pressing force, the counterforce applying mechanism needs to be disposed so as to apply the counterforce in the direction of the pressing force. Therefore, the degree of freedom of arrangement of the counterforce applying mechanism is small and, in some cases, the size of the internal combustion engine in the direction of the pressing force increases.
When the counterforce applying mechanism of the torque reducing mechanism is in contact with the counteraction cam when the camshaft is at an angular position where the torque fluctuates in a narrow range, friction between the counterforce applying mechanism and the counteraction cam increases loss in the torque for rotating the camshaft